1. Field of the Invention
The invention in general relates to the fabrication of layered superlattice materials and ABO.sub.3 type metal oxides, and more particularly to a method for fabricating ferroelectric integrated circuits that protects the elements of ferroelectric devices from damage by hydrogen.
2. Statement of the Problem
Ferroelectric compounds possess favorable characteristics for use in nonvolatile integrated circuit memories. See Miller, U.S. Pat. No. 5,046,043. A ferroelectric device, such as a capacitor, is useful as a nonvolatile memory when it possesses desired electronic characteristics, such as high residual polarization, good coercive field, high fatigue resistance, and low leakage current. Lead-containing ABO.sub.3 type ferroelectric metal oxides such as PZT (lead titanate zirconate) and PLZT (lanthanum lead titanate zirconate) have been studied for practical use in integrated circuits. Layered superlattice materials have also been studied for use in integrated circuits. See Watanabe, U.S. Pat. No. 5,434,102. Layered superlattice material compounds exhibit characteristics in ferroelectric memories that are orders of magnitude superior to those of PZT and PLZT compounds. While prototypes of ferroelectric memories have been made successfully with the layered superlattice compounds, there is as yet no manufacturing process for making memories using either the ABO.sub.3 type oxides or the layered superlattice material compounds with the desired electronic characteristics economically and in commercial quantities. One reason, among others, for the lack of economical commercial processes for the fabrication of high quality ferroelectric integrated circuits is that the metal oxide compounds are susceptible to reduction by hydrogen during hydrogen annealing. Hydrogen annealing is a common step during CMOS integrated circuit memory fabrication and results in degradation of some important ferroelectric properties. This is especially true for the layered superlattice material compounds, which are complex, layered oxides that are especially prone to degradation by hydrogen.
A typical ferroelectric memory device in an integrated circuit contains a semiconductor substrate and a metal-oxide semiconductor field-effect transistor (MOSFET) in electrical contact with a ferroelectric device, usually a ferroelectric capacitor. A ferroelectric capacitor typically contains a ferroelectric thin film located between a first, bottom electrode and a second, top electrode, the electrodes typically containing platinum. During manufacture of the circuit, the MOSFET is subjected to conditions causing defects in the silicon substrate. For example, the manufacturing process usually includes high energy steps, such as ion-mill etching and plasma etching. Defects also arise during heat treatment for crystallization of the ferroelectric thin film at relatively high temperatures, often in the range 500.degree.-900.degree. C. As a result, numerous defects are generated in the single crystal structure of the semiconductor silicon substrate, leading to deterioration in the electronic characteristics of the MOSFET.
To restore the silicon properties of the MOSFET/CMOS, the manufacturing process typically includes a hydrogen annealing step, in which defects such as dangling bonds are eliminated by utilizing the reducing property of hydrogen. Various techniques have been developed to effect the hydrogen annealing, such as H.sub.2 -gas heat treatment in ambient conditions. Conventionally, hydrogen treatments are conducted between 350.degree. and 550.degree. C., typically around 450.degree. C. for a time period of about 30 minutes. In addition, there are several other integrated circuit fabrication processes that expose the integrated circuit to hydrogen, often at elevated temperatures, such as CVD processes for depositing metals, and growth of silicon dioxide from silane or TEOS sources. During processes that involve hydrogen, the hydrogen diffuses through the top electrode and the side of the capacitor to the ferroelectric thin film and reduces the metal oxides contained in the ferroelectric material. The absorbed hydrogen also metallizes the surface of the ferroelectric thin film. The adhesivity of the ferroelectric thin film to the upper electrode is lowered by the chemical change taking place at the interface. Alternatively, the upper electrode is pushed up by the oxygen gas, water, and other products of the oxidation-reduction reactions taking place. As a result of these effects, the electronic properties of the capacitor are degraded, and peeling is likely to take place at the interface between the top electrode and the ferroelectric thin film. In addition, hydrogen also can reach the lower electrode, leading to internal stresses that cause the capacitor to peel off its substrate. These problems are acute in ferroelectric memories containing layered superlattice compounds because these metal oxide compounds are particularly complex and prone to degradation by hydrogen-reduction.
3. Solution to the Problem
The present invention provides a process for fabricating thin films of layered superlattice materials and ABO.sub.3 type metal oxides for use in integrated circuits that reduces the detrimental effects of the hydrogen heat treatment and preserves the favorable electronic properties of the ferroelectric element.
One aspect of the invention is an H.sub.2 -gas treatment step conducted at relatively low temperatures, in the range 200.degree.-350.degree. C.
Another aspect of the invention is that the H.sub.2 -treatment is conducted for a short time period, less than 30 minutes.
The relative concentration of hydrogen gas is in the range 0.01 to 50 percent. The heat treatment is ambient tolerant so that it can be conducted, for example, at atmospheric pressure and in the presence of nitrogen or other inert gases.
In a preferred method, a nitride of titanium or silicon is formed to cover the ferroelectric element and serve as a hydrogen barrier.
A further aspect of the invention is forming an integrated circuit element containing a thin film of oxide material containing at least two metals. In a preferred embodiment, the thin film comprises a layered superlattice compound. Preferably, at least one of the metals is present in the oxide material in an excess amount. A further aspect of the invention is forming ferroelectric material in which at least one of the constituent metals is present in amounts sufficient to form separate metal oxides in the ferroelectric material. In one embodiment of the invention the layered superlattice compound comprises strontium bismuth tantalum niobate. Another aspect of the invention is a method wherein the metal present in an excess amount is either niobium or bismuth. A further aspect of the invention is forming a metal oxide element having layered superlattice compounds containing the chemical elements bismuth, niobium and tantalum in which the relative amounts of the chemical elements are selected to minimize the degradation of electronic properties by hydrogen.
Another aspect of the invention is providing a substrate and a precursor liquid for the oxide material, applying the precursor to the substrate and treating it to form the oxide material. Another aspect of the invention is that the oxide material comprises an oxide compound containing at least two metals. A further aspect of the invention is that at least one of the metals is present in the precursor in an excess amount. A still further aspect of the invention is that a metal present in an excess amount is a metal that does not form volatile compounds that dissipate during said fabrication process.
Another aspect of the invention is that the oxide material comprises strontium bismuth tantalum niobate and the precursor contains the chemical elements strontium, bismuth, tantalum and niobium having relative molar proportions corresponding approximately to the stoichiometric formula SrBi.sub.2.18 Ta.sub.2-x Nb.sub.x O.sub.9, where 0.ltoreq..times..ltoreq.2. In one embodiment, the precursor contains an additional amount of bismuth corresponding to between zero percent and 40 percent above the stoichiometric amount represented by the formula SrBi.sub.2.18 Ta.sub.2-x Nb.sub.x O.sub.9, where 0.ltoreq..times..ltoreq.2. In one embodiment, the precursor contains relative molar proportions of the elements strontium, bismuth, tantalum and niobium corresponding approximately to the formula SrBi.sub.2.18 Ta.sub.1.44 Nb.sub.0.56 O.sub.9. In a preferred embodiment, layered superlattice material contains an additional amount of niobium corresponding to between zero percent and 40 percent above the stoichiometric amount represented by the formula SrBi.sub.2.18 Ta.sub.1.44 Nb.sub.0.56 O.sub.9.
A further aspect of the invention is performing an oxygen-gas recovery anneal at a temperature range from 300.degree. to 1000.degree. C. for a time period from 20 minutes to 2 hours to restore desired electronic properties that degrade during hydrogen treatments.
The invention provides a process and IC structure that permits ferroelectric electronic elements, particularly elements made with the layered superlattice material compounds, to be made in commercial quantities without significant degradation of their electronic properties. Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.